The formation of NMR images based upon excited nuclei is well known in the art. A typical sequence consists of a plurality of contiguous T.sub.R time periods. In the case of multi-slice/multi-echo sequence, each T.sub.R time period is further divided into a plurality of slice time intervals, or subinterval time periods.
In a serial exciting sequence, a single physical slice is excited and echo signals are acquired, in each subinterval time period. The echo signal is received by subjecting the patient or subject under study to a particular magnetic gradient pulse and applying a single RF signal having a particular frequency during the time period in which the magnetic gradient pulse is applied. The combination of the RF signal at the particular frequency and the application of the magnetic gradient pulse, serves to selectively excite a particular physical slice of selected nuclei. The readout of the RF signal occurs by the application of another magnetic gradient during which time the RF signal is received from the excited nuclei, as it decays, in the selected slice.
In an effort to expedite the acquisition of RF signals from a plurality of physical slices, U.S. Pat. No. 4,843,322 discloses a method for simultaneously exciting a plurality of slices in each single subinterval time periods to produce multi-slice NMR images. That reference discloses the application of two RF excitation pulses having different frequency during the subinterval time period in which a magnetic gradient is applied. (See column 10, lines 53-55). The application of RF signals simultaneously requires a considerable amount of power to the RF transmitter in the MRI apparatus. Therefore, as a practical matter, the teaching of the reference U.S. Pat. No. 4,843,322 has been limited to the application of two RF pulses simultaneously, thereby exciting only two physical slices simultaneously.
Once the plurality of RF echo signals from each subinterval time periods are received, U.S. Pat. No. 4,843,322 discloses the method of processing it in order derive the plurality of slice images from the plurality of physical slices. In addition, see "SIMA: Simultaneous Multi-slice Acquisition of MR Images by Hadmard-encoded Excitation" S. P. Souza et al, Journal of Computer Assisted Tomography, 12, 1026 (1988), referenced in U.S. Pat. No. 4,843,322 for disclosing the processing of the multi-signals acquired to produce multi-images.